ENH: Only classes belonging to API are now contained in 4neuro.h. All those classes were enclosed in 'lib4neuro' namespace.

ADD: added preliminary version of gradient calculation vie backpropagation, added supporting methods to error classes and learning algorithms utilizing gradient calculation

ENH: Removed both linking and include dependencies from examples => lib4neuro can be used just by linking it and including its header without any external dependencies needed.

FIX: Removed included Exprtk - now it's compiled as ExprtkWrapper and it's not needed to include the whole header-library.

FIX: fixed an issue with ExprtkWrapper being incorrectly destryed before being used in the NetworkSum class

MOD: Modified examples and unit tests to account for the changes